1. Field of the Invention
This invention relates to a surface state inspection apparatus, and more particularly, to a surface state inspection apparatus which is suitable for detecting foreign particles or pattern defects on a pattern or on a surface of a photomask, a reticle or the like (hereinafter generically termed a reticle), serving as an original plate for pattern transfer in a semiconductor exposure apparatus.
2. Description of the Prior Art
In general, in integrated circuit (IC) production processes, a circuit pattern for exposure formed on a reticle substrate is transferred onto the surface of a wafer coated with a resist using a semiconductor printing apparatus (a stepper or a mask aligner), to produce ICs.
At that time, if foreign particles, such as dust or the like, are present on the surface of the substrate, the foreign particles are simultaneously transferred, causing a decrease in the yield of the IC production.
Particularly, when a plurality of circuit patterns are printed on a wafer by a step-and-repeat method using a reticle, one foreign particle on the reticle is printed on the entire surface of the wafer, greatly decreasing the yield of the IC production. Accordingly, it is indispensable to detect the presence of foreign particles on a substrate in an IC production process, and various inspection methods have been proposed. FIG. 1 is an example of such a method. In FIG. 1, a laser beam emitted from a laser light source 1 is expanded by a beam expander 2. Subsequently, the laser beam is condensed onto a reticle 5 by an f-.theta. lens 4 while the reticle 5 is being scanned by a polygon mirror 3 (or a light scanning element represented by a vibrating mirror). The beam scanning direction is perpendicular to the plane of FIG. 1. By performing stage scanning in the directions shown by double-headed arrows S.sub.1 and S.sub.2 within the plane of FIG. 1 (the direction orthogonal to the beam scanning direction), the inspection beam is projected (raster scanning) onto the entire surface of the reticle 5.
If a foreign particle is present on point O.sub.0 on the reticle 5 during the beam scanning, a portion of the light diffused by the foreign particle also returns to the incident side (the right side of FIG. 1) of the beam (backscattering light). A mirror 20 is provided for reflecting the scattered light from point O.sub.0, and the point O.sub.0 is reimaged on a field stop 22 using a condenser lens 21. The field stop 22 is provided in order to cut flare light from portions other than the point O.sub.0, and includes a rectangular aperture (the direction of its longer side coincides with the scanning direction of the beam), as shown in FIG. 2. Scattered light beams passing through this aperture diffuse again, but are condensed by a condenser lens 23, and are received by a photomultiplier 24.
The optical relationship among the light-issuing point O.sub.0, the condenser lens 21 and the field stop 22 will now be explained. The diffused light beams from the point O.sub.0 are reflected by the mirror 20. If a virtual image O'.sub.0 of the point O.sub.0 produced by the mirror 20 is considered, it can be deemed that the light beam emanating from the point O'.sub.0 is imaged on a point I.sub.0 on the image surface by the condenser lens 21. The field stop 22 is arranged so that its aperture coincides with the point I.sub.0.
In receiving scattered light, it is desirable that a foreign particle which has a peculiarity in the scattering direction can also be detected. The configuration of the conventional apparatus has a disadvantage that, for the above-described purpose, and in order to increase the amount of detected light, if it is intended to receive the scattered light from the point O.sub.0 with a wide receiving angle (angle .theta..degree. in FIG. 1), the aperture of the condenser lens 21 must be greatly increased. Particularly in an optical system as shown in FIG. 1 wherein scanning of an object to be inspected is performed, a long operational distance (O.sub.0 -C+C-P in FIG. 1) for the scanning must be provided, increasing the aperture of the condenser lens 21. For example, if it is assumed that the operational distance equals 300 mm and the receiving angle for the light beam .theta..degree. equals 30.degree., the diameter of the condenser lens in the plane of FIG. 1 becomes as large as EQU 300 mm.times.tan (30.degree./2).times.2.apprxeq.160 mm.
A most widely used inspection apparatus of this kind is an apparatus for inspecting foreign particles on a reticle mounted on a semiconductor printing apparatus (stepper). Also, in this apparatus, increasing the amount of scattered light from a foreign particle as much as possible and widening the angle for receiving the light are necessary conditions for increasing the detection rate for foreign particles. On the other hand, making an inspection unit as small as possible is an important factor in order to mount such an apparatus on a stepper. In the conventional apparatus, it has been difficult to satisfy the above-described contradictory requirements. That is, the conventional apparatus has a disadvantage that, if the size of the inspection unit is reduced, the size of the condenser lens 21 is inevitably reduced. The amount of received scattered light from an object to be inspected and the angle for receiving the scattered light are thereby reduced, and hence, sufficient detection capability cannot be obtained.